The present invention relates to a polyester film to belaminated onto a metal plate-and molded. More specifically, it relates to a polyester to be laminated onto a metal plate and molded, which exhibits excellent moldability when laminated onto a metal plate to be subjected to a can making process such as drawing and which can be used to produce metal cans having excellent heat resistance, retort resistance, taste-and-flavor retainabilities and impact resistance, such as drink cans and food cans.
Metal cans are generally coated on interior and exterior surfaces to prevent corrosion. Recently, the development of methods for obtaining corrosion resistance without using an organic solvent has been promoted for the purpose of simplifying production process, improving sanitation and preventing pollution. One of the methods is to coat a metal can with a thermoplastic resin film.
That is, studies on a method for making cans, which comprises laminating a thermoplastic resin film on a plate of a metal such as tin, tin-free steel or aluminum and drawing the laminated metal plate, are under way.
It gradually becomes clear that a copolyester film is suitable for use as this thermoplastic resin film in terms of moldability, heat resistance, impact resistance and taste-and-flavor retainabilities. This polyester film, however, does not always exhibit sufficient taste-and-flavor retainabilities when a can coated therewith contains a drink whose delicate taste is important, such as green tea, or mineral water which must be tasteless and odorless, and changes in flavor and taste of the contents are detected.
JP-A 6-116376 proposes a polyester film to be laminated onto a metal plate and molded, which is made from a copolyester containing alkali-metal elements and a germanium element in specific amounts and which has improved flavor retainabilities When this film is used to coat a can, however, it exhibits excellent taste-and-flavor retainabilities as in a cold pack system in which heat is not applied to the can with contents, whereas it does not always obtain sufficient taste-and-flavor retainabilities as in a retort treatment in which heat is applied to the can with contents.
It is an object of the present invention to provide a polyester film to be laminated onto a metal plate and molded, which solves the above problems of the prior art and which has improved taste-and-flavor retainabilities, particularly taste-and-flavor retainabilities after a retort treatment without losing excellent moldability, heat resistance, impact resistance and retort resistance of a copolyester film.
The other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, the above objects and advantages of the present invention are attained by a biaxially oriented polyester film to be laminated onto a metal plate and molded, (A) which comprises a copolyester comprising (a) terephthalic acid in an amount of 82 to 100 mol % and 2,6-naphthalenedicarboxylic acid or a combination of 2,6-naphthalenedicarboxylic acid and other dicarboxylic acid in an amount of 0 to 18 mol % of the total of all dicarboxylic acid components and (b) ethylene glycol in an amount of 82 to 100 mol % and cyclohexanedimethanol or a combination of cyclohexanedimethanol and other diol in an amount of 0 to 18 mol % of the total of all diol components, having (c) a glass transition temperature of 78xc2x0 C. or more and (d) a melting point of 210 to 250xc2x0 C., and containing (e) porous silica particles with a pore volume of 0.5 to 2.0 ml/g which are agglomerates of primary particles having an average particle diameter of 0.001 to 0.1 xcexcm; and
(B) which has the following relationship between the highest peak temperature (Te, xc2x0C.) of loss elastic modulus and the glass transition temperature (Tg, xc2x0C.):
Texe2x88x92Tgxe2x89xa630.
The copolyester in the present invention comprises terephthalic acid in an amount of 82 to 100 mol % and 2,6-naphthalenedicarboxylic acid or a combination of 2,6-naphthalenedicarboxylic acid and other dicarboxylic acid in an amount of 0 to 18 mol % of the total of all dicarboxylic acid components.
Illustrative examples of the other dicarboxylic acid include aromatic dicarboxylic acids such as isophthalic acid and phthalic acid; aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid and decanedicarboxylic acid; and alicyclic dicarboxylic acids such as cyclohexanedicarboxylic acid. They may be used alone or in combination of two or more.
The copolyester in the present invention comprises ethylene glycol in an amount of 82 to 100 mol % and cyclohexanedimethanol or a combination of cyclohexanedimethanol and other diol in an amount of 0 to 18 mol % of the total of all diol components.
Illustrative examples of the other diol include aliphatic diols such as diethylene glycol, propylene glycol, neopentyl glycol, butanediol, pentanediol and hexanediol; alicyclic diols such as cyclohexanedimethanol; aromatic diols such as bisphenol A; and polyalkylene glycols such as polyethylene glycol and polypropylene glycol. They may be used alone or in combination of two or more.
The above copolyester may comprise at least one or both of 2,6-naphthalenedicarboxylic acid and 1,4-cyclohexanedimethanol as a copolymer component.
It is particularly preferable that all the dicarboxylic acid components of the copolyester consist of terephthalic acid and 2,6-naphthalenedicarboxylic acid and that all the diol components of the copolyester consist of ethylene glycol.
The copolyester in the present invention has a glass transition temperature (Tg) of 78xc2x0 C. or more and a melting point of 210 to 250xc2x0 C.
If Tg is lower than 78xc2x0 C., heat resistance will deteriorate and taste-and-flavor retainabilities after a retort treatment will degrade when the film of the present invention is laminated onto a metal plate and molded into a metal can. To increase the Tg of the copolyester of the present invention to 78xc2x0 C. or higher, 2,6-naphthalenedicarboxylic acid and cyclohexanedimethanol are used as copolymer components.
The glass transition temperature (Tg) of the copolyester is preferably in the range of 78 to 90xc2x0 C.
To obtain a Tg of polyester, a 20-mg film sample is placed in a DSC measurement pan, molten by heating on a stage at 290xc2x0 C. for 5 minutes and solidified by quenching the pan on an aluminum foil laid on ice to obtain a glass transition point at a temperature elevation rate of 20xc2x0 C./min using the 910 DSC of Du Pont Instruments.
When the melting point is lower than 210xc2x0 C., the heat resistance of the polymer deteriorates. On the other hand, when the melting point is higher than 250xc2x0 C., the crystallinity of the polymer becomes too high with the result of impaired moldability.
The melting point of the copolyester is preferably in the range of 210 to 245xc2x0 C.
The melting point of copolyethylene terephthalate is measured in accordance with a method for obtaining a melting peak at a temperature elevation rate of 20xc2x0 C./min using the 910 DSC of Du Pont Instruments. The quantity of a sample is 20 mg.
The intrinsic viscosity (orthochlorophenol, 35xc2x0 C.) of the copolyester is preferably in the range of 0.52 to 1.50, more preferably 0.57 to 1.00, particularly preferably 0.60 to 0.80. When the intrinsic viscosity is lower than 0.52, impact resistance may be insufficient disadvantageously. On the other hand, when the intrinsic viscosity is higher than 1.50, moldability may be impaired.
The content of acetaldehyde in the copolyester is preferably 15 ppm or less, more preferably 12 ppm or less, much more preferably 10 ppm or less.
When the content of the acetaldehyde is larger than 15 ppm, the taste-and-flavor retainabilities of the contents tend to lower disadvantageously.
The concentration of the terminal carboxyl groups of the copolyester is preferably 40 equivalents/106 g or less, more preferably 35 equivalents/106 g or less, much more preferably 30 equivalents/106 g.
When the concentration of the terminal carboxyl groups is higher than 40 equivalents/106 g, the amount of the acetaldehyde contained in the film tends to increase as well and the taste-and-flavor retainabilities of the contents are apt to lower. Heat resistance and retort resistance are also liable to lower and excellent properties obtained by the present invention are canceled disadvantageously.
The electric resistivity at 290xc2x0 C. of the molten copolyester is preferably set to 5xc3x97106 to 1xc3x97109 xcexa9xc2x7cm to achieve not only excellent flatness by employing an electrostatic impression process when the film of the present invention is produced but also excellent lamination property and moldability when the film is laminated onto a metal plate and molded into a metal can. When the electric resistivity is lower than 5xc3x97106 xcexa9xc2x7cm, the taste-and-flavor retainabilities after can making deteriorate disadvantageously. On the other hand, when the electric resistivity is higher than 1xc3x97109 xcexa9xc2x7cm, film productivity lowers and lamination property and moldability deteriorate disadvantageously.
Although the copolyester in the present invention is not limited by a production process thereof, preferred processes for producing a desired copolyester are one which comprises subjecting terephthalic acid, ethylene glycol and a copolymer component to an esterification reaction and polycondensing the reaction product until a target degree of polymerization is achieved, and one which comprises subjecting dimethyl terephthalate, ethylene glycol and a copolymer component to an ester interchange reaction and polycondensing the reaction product until. a target degree of polymerization is achieved. The copolyester obtained by one of the above processes (melt polymerization) can be changed into a polymer having a higher degree of polymerization by polymerization in asolid phase (solid-phase polymerization) as required.
The copolyester may contain such additives as an antioxidant, heat stabilizer, viscosity modifier, plasticizer, color modifier, lubricant, nucleating agent and ultraviolet absorber as required.
Preferred examples of the catalyst used for the above polycondensation reaction include antimony compounds (Sb compounds), titanium compounds (Ti compounds) and germanium compounds (Ge compounds). Of these, titanium compounds and germanium compounds are more preferred from the viewpoint of the flavor retainabilities of a film. Preferred titanium compounds include titanium tetrabutoxide and titanium acetate. Preferred germanium compounds include (a) amorphous germanium oxide, (b) fine crystalline germanium oxide, (c) a solution of germanium oxide dissolved in glycol in the presence of an alkali metal, alkaline earth metal or compound of these and (d) a solution of germanium oxide dissolved in water. When an antimony compound and a titanium compound are used in combination, taste-and-flavor retainabilities can be improved and costs can be reduced advantageously.
The copolyester must contain porous silica particles which are agglomerated particles. When only globular or amorphous silica particles are contained as in the prior art, a remarkable effect of improving taste-and-flavor retainabilities cannot be obtained.
The average particle diameter of primary particles forming the porous silica particles must be in the range of 0.001 to 0.1 xcexcm. When the average particle diameter of the primary particles is smaller than 0.001 xcexcm, very fine particles are produced by cracking in the stage of a slurry and form agglomerates, causing the formation of pin holes with the result of deterioration in moldability. On the other hand, when the average particle diameter of the primary particles is larger than 0.1 xcexcm, the porosity of the particles is lost with the result that taste-and-flavor retainabilities are not improved.
Further, the pore volume of the porous silica particles must be in the range of 0.5 to 2.0 ml/g, preferably 0.6 to 1.8 ml/g. When the pore volume is smaller than 0.5 ml/g, the porosity of the particles is lost with the result that taste-and-flavor retainabilities are not improved. On the other hand, when the pore volume is larger than 2.0 ml/g, agglomeration readily occurs by cracking, causing the formation of pin holes with the result of deterioration in moldability.
The particle diameter and amount of the porous silica particles may be determined according to film winding property, pin hole resistance and taste-and-flavor retainabilities. The average particle diameter of the porous silica particles is generally in the range of 0.1 to 5 xcexcm, preferably 0.3 to 3 xcexcm, and the amount thereof is generally in the range of 0.01 to 1 wt %, preferably 0.02 to 0.5 wt %.
Although the porous silica particles used in the present invention are agglomerated, the polyester film of the present invention preferably contains coarse agglomerated particles whose size is 50 xcexcm or more at a density of 10/m2 or less, more preferably 5/m2 or less, much more preferably 3/m2 or less. When the number of coarse agglomerated particles whose size is 50 xcexcm or more is too large, pin holes are readily formed and moldability is apt to deteriorate.
To reduce the number of coarse agglomerated particles, it is preferable to filter a molten polymer using a non-woven filter, which is formed of a thin stainless steel wire having a diameter of 15 xcexcm or less and which has an average mesh size of 10 to 30 xcexcm, preferably 15 to 25 xcexcm, as a filter for the production of a film. The porous silica particles are generally added to a reaction system, preferably as a slurry contained in a glycol, at the time of a reaction for producing a polyester, for example, at any time during an ester interchange reaction or a polycondensation reaction when an ester interchange method is employed or at any time when a direct polymerization method is employed. It is particularly preferable that the porous silica particles be added to the reaction system in the initial stage of the polycondensation reaction, for example, before the intrinsic viscosity reaches about 0.3.
A lubricant is preferably added to the copolyester for the purpose of improving film winding property. The lubricant may be inorganic or organic but preferably inorganic. Illustrative examples of the inorganic lubricant include silica, alumina, titanium oxide, calcium carbonate and barium sulfate, and illustrative examples of the organic lubricant include silicone resin particles and crosslinked polystyrene particles. The lubricant is preferably monodisperse inert spherical particles, which have a particle diameter ratio (long diameter/short diameter) of 1.0 to 1.2 and which are not substantially agglomerated, particularly from the viewpoint of pin hole resistance. Illustrative examples of such a lubricant include completely spherical silica, completely spherical silicone resin particles, and spherical crosslinked polystyrene.
The average particle diameter of the inert spherical particles is preferably 2.5 xcexcm or less, more preferably 0.05 to 1.5 xcexcm.
In the present invention, the average particle diameter of the inert spherical particles is particularly preferably smaller than the average particle diameter of the above porous silica particles and in the range of 0.05 to 0.8 xcexcm.
The content of the inert spherical particles is preferably 0.01 to 1 wt %.
The lubricant is not limited to the above externally added particles and may be internally deposited particles obtained by depositing part or all of the catalyst used in the production of a polyester in a reaction step, for example. It is also possible to use the externally added particles and the internally deposited particles in combination.
Two different kinds of particles having different average particle diameters may be used in combination as the lubricant or the inert spherical particles.
The biaxially oriented polyester film of the present invention is made from the above copolyester having the following relationship between the highest peak temperature (Te, xc2x0C.) of loss elastic modulus and the glass transition temperature (Tg, xc2x0C.).
Texe2x88x92Tgxe2x89xa630
When the value of Texe2x88x92Tg is larger than 30, the molecule orientation and crystallinity of the film become too high, with the result of great deterioration in moldability. The value of Te, which depends on the type and amount of the copolymer component, is preferably adjusted particularly by the stretch ratios of biaxial stretching, stretching temperature and heat-setting temperature according to film formation conditions.
Te is obtained at a measurement frequency of 10 Hz and a dynamic displacement of xc2x125xc3x9710xe2x88x924 cm using a dynamic visco-elastometer.
The relationship between the highest peak temperature (Te) of loss elastic modulus and the glass transition temperature (Tg) is preferably
15xe2x89xa6Texe2x88x92Tgxe2x89xa625.
The refractive index in a thickness direction of the polyester film of the present invention is preferably 1.500 to 1.545, more preferably 1.505 to 1.530. When this refractive index is too low, moldability becomes unsatisfactory. On the other hand, when the refractive index is too high, the structure of the polyester film becomes almost amorphous, whereby heat resistance may lower.
The refractive index in a thickness direction of the polyester film is measured by a monochromatic NaD ray with a polarizing plate analyzer attached to the eyepiece side of an Abbe""s refractometer. The mount solution is methylene iodide and the measurement temperature is 25xc2x0 C.
The center line average roughness (Ra) of the polyester film surface of the present invention is preferably 35 nm or less from the viewpoints of film winding property and taste-and-flavor retainabilities. Ra is more preferably 15 nm or less, particularly preferably 4 to 15 nm.
The center line average roughness (Ra) of the film surface is measured in accordance with JIS-B0601 and defined as a value (Ra: nm) obtained from the following expression when a portion having measurement length L is extracted from a film surface roughness curve in its center line direction, the center line of the extracted portion is taken as an X axis and the direction of the longitudinal stretch ratio is taken as an Y axis to represent a roughness curve Y=f(x).
Ra=1/L∫0L|f(x)|dx
In the present invention, five portions having a reference length of 2.5 mm are measured and the mean of four measurement values excluding the largest value is taken as Ra.
Since the polyester film of the present invention is used especially in food cans and drink cans, it is preferable that the amount of a substance dissolved out or dispersed from the film be as small as possible. However, it is substantially impossible to eliminate the substance. Therefore, to use the polyester film of the present invention in food or drink cans, the amount of the film extracted with ion exchange water at 121xc2x0 C. for 2 hours is preferably 0.5 mg/cm2 or less (0.0775 mg/cm2 or less), more preferably 0.1 mg/cm2 or less (0.0155 mg/cm2 or less).
To reduce the amount of the extracted film, it is recommended to increase Tg of the copolyester.
The polyester film of the present invention preferably has a thickness of 6 to 75 xcexcm, more preferably 8 to 75 xcexcm, particularly preferably 10 to 50 xcexcm. When the thickness is smaller than 6 xcexcm, the polyester film is easily broken at the time of processing. On the other hand, when the thickness is larger than 75 xcexcm, the polyester film has excessive quality, which is uneconomical.
The metal plate to be laminated with the polyester film of the present invention, particularly a metal plate for can making is advantageously a plate of tin, tin-free steel, aluminum or the like. The polyester film can be laminated on the metal plate by the following methods (1) and (2), for example.
(1) The metal plate is heated to a temperature higher than the melting point of the film, laminated with the film and quenched. This makes the surface layer portion (thin layer portion) of the film, which is in contact with the metal plate, amorphous, whereby the film is bonded to the metal plate.
(2) A primer is coated on the film to form an adhesive layer and the film is laminated on the metal plate in such a manner that the adhesive layer comes into contact with the metal plate. Known resin adhesives such as epoxy adhesives, epoxy-ester adhesives and alkyd adhesives can be used to form the adhesive layer.